This invention relates to a frequency stabilizing device for a laser diode module.
Laser diodes are used in optical communication, optical information processing, optical measurement, and various other applications of optics. For use in such a field of application, the laser diode is included in a module which is usually called a laser diode module and will later be exemplified. The laser diode module is therefore for generating a laser beam along an optical path. The laser beam has a frequency or wavelength which is inevitably subjected to a variation. Frequency stabilization of a laser beam is therefore urgently required.
On stabilizing the frequency of a laser beam, a frequency reference is used in detecting a frequency difference between the frequency of the laser beam and the frequency reference. An error signal is produced from the frequency difference for use in giving negative feedback to the laser diode module to control the temperature of the laser diode, an injection current supplied to the laser diode, or both.
In a conventional frequency stabilizing device for a laser diode module, a Fabry-Perot resonator is used as the frequency reference. An example of such conventional frequency stabilizing devices is described in a letter contributed by M. S. Nakamura and another to the Electronics Letters, Volume 26, No. 6 (Mar. 15, 1990), pages 405 and 406, under the title of "Frequency-Stabilised LD Module with a Z-Cut Quartz Fabry-Perot Resonator for Coherent Communication".
It has, however, been impossible by a frequency stabilizing device comprising the Fabry-Perot resonator by itself to get a zero-cross signal as the error signal. Generally speaking, the zero-cross signal should represent zero when a predetermined point is reached by a quantity to be controlled. The zero-cross signal should have a positive and a negative value when the quantity respectively has positive and negative errors relative to the predetermined point. Moreover, the laser beam is subjected to a considerable amount of reflection on entering the Fabry-Perot resonator. The conventional frequency stabilizing devices have therefore been complicated, bulky, expensive, and unreliable.